Question: On the first 4 quizzes of her math class, Ashley got an average score of 79. What does she need on the next quiz to have an overall average of 81?
Solution: Let her score on the next quiz be $x$ The sum of all of her scores is then $4 \cdot 79 + x$ The same sum must also be equal to $5 \cdot 81$ Solve: $x = 5 \cdot 81 - 4 \cdot 79 = 89$.